Primer: How to Read a Barometer

These days, we take weather forecasts for granted. Simply opening our trusty weather apps or catching the latest news forecast during our local news hour is borderline mindless. Often, we’re told the forecast typically up to seven days in advance with little to no understanding of how or why it’s going to rain on Tuesday and once again be sunny on Wednesday. Instead, we find ourselves complaining about a potential washout or an early season chill – blaming naturally occurring phenomena or the meteorologists themselves for blowing a forecast rather than studying weather trends or atmospheric patterns to better understand our environment

It’s the level of convenience we have today that serves to sort of propagate the apparent apathy toward grasping what’s happening in the atmosphere around us. However, it didn’t use to be that way. For it was the miracle of human curiosity that first led to the discovery of what can be considered the “great dictator” of the earth’s weather: atmospheric pressure. We’ve all witnessed the symptoms of atmospheric pressure, and most likely have even seen our local barometric reading when checking local weather conditions without fully understanding exactly what they meant. Well, understanding barometric pressure has been a way weather forecasters were first able to predict conditions in a specific area using only one tool: the barometer. The best part? These tools are quite easy to read with a little know-how.

Air Pressure

An Introduction

Anyone who’s looked at a weather map typically notices two features above all else – the seemingly random placement of high pressure “H” and low pressure “L” across the map. Well, it may come as a surprise but these are not random. In fact, they mark the locations of the lowest and highest pressure vortexes within a given area. As an aside, it’s worth noting atmospheric pressure is simply the weight of the air at ground level so it’s understandable pressures will always be higher at sea level than on a mountaintop since there is less air weighing down upon the earth’s surface.

As you can see, the atmosphere is constantly shifting from low pressure to high pressure, leading to changes in the weather patterns. Also, depending on the location, residents in the US can witness some wild transitions in weather sometimes over the course of a few hours depending on how close these pressure vortexes are to one another.

Moving back to the weather map, high pressure in this instance denotes heavy sinking air diving from the upper levels of the atmosphere to the earth’s surface, spreading out evenly over the course of a given distance (oftentimes hundreds of miles). Low pressure, on the other hand, represents a lack of sinking air over a given area. In fact, quite the opposite is true. Here, these lower pressures result in rising air. And since warm air at the surface naturally wants to rise, cool, and condense in the atmosphere to form precipitation, cloudy and stormy weather are almost always associated with low-pressure systems.

So, since high pressure denotes sinking air, cool air aloft is pushed down to the earth’s surface and is then warmed by the sun. Therefore, it’s typical to have few clouds, sunny skies, and dry air with high-pressure systems.

Barometers

Three Types, Three Eras

So, how can you use this information to your advantage? Well, utilizing a barometer you can actually forecast conditions in your local area for up to 48 hours in advance based on air pressure trends. It’s how, before we had smartphone apps and professional meteorologists at our disposal, individuals were able to predict weather conditions – which was especially handy for farmers, hunters, or practically anyone who made a living outside. From here, there are three standard options to choose from, which we’ll dive into now.

Three Types of Barometers

We’ve come a long way since the early days of measuring barometric pressure. However, the idea and methods remain same because, in the end, it’s all about measuring the weight of the atmosphere around us.

Mercury

It’s was one of Galileo’s students by the name of Evangelista Torricelli who, in 1640, first invented the barometer utilizing a long tube, mercury, and a cistern. Here, the air presses down on the open mercury in the cistern, raising or lowering that amount of mercury still left in the tube. However, since mercury isn’t exactly the safest substance out there, very few mercury-based barometers are available today.

Aneroid

Here, we have a trusty analog version that’s most likely the type of barometer most recognizable as a cylindrical instrument commonly mounted on the kitchen walls of old farmhouses or sailing vessels. This is a non-liquid device that uses what’s referred to as an aneroid cell that’s made from an alloy of beryllium and copper. In this instance, changes in air pressure cause the box to either expand or contract thus driving its mechanical levers on the dial face accordingly. These are often beautiful instruments that can tie an at-home weather station together with finesse.

Digital

The newest iterations available, these are the barometers of the modern era. They utilize what’s called a strain gauge – a thin wire that reacts to changes in electrical resistance due to alterations in atmospheric pressure acting on a diaphragm that’s wrapped around it. What results is an easy to read interface that not only provides you with the current pressure and whether it’s rising or falling, but the expected weather conditions associated with that pressure.

How to Read

Setting Up for Success

This brings us to the main event – how to read a barometer – since without knowledge of what these numbers mean the instrument is rendered useless. For this example, we’ll utilize the Aneroid since there isn’t much set-up involved with the electronic option. So, before using your Aneroid barometer, you’re going to need to calibrate the piece. This can be done with a small screwdriver to set the hand to your current barometric pressure (easily found online). To do this simply turn the small screw, typically located at the bottom right on the backside of the barometer accordingly.

From here, it’s worth noting that here in the US we measure pressure in either millibars (metric) or inches (standard) ranging between 28 and 31 inches or 960 and 1080 millibars (though there have been some outlying instances). It’s also worth noting that millibars are simply a conversion from inches, so with a little arithmetic you can easily toggle back and forth between the two measurements.

Now, once the barometric pressure is set to your current conditions, set the second hand (manual hand) to align with the initial hand you just set. Now, any changes in air pressure (movement of the automatic hand) will be made apparent by the new difference between that and the manual hand you just aligned with it.

General Guidelines

Typically, barometric pressure is measured in either inches or millibars. And just like any other forms of measurement, you can convert one to another. Some barometers, however, feature both – which is often what we find on the dial face of a standard Aneroid barometer.

Finally, when it comes to reading and understanding the difference, here’s a general rule of thumb from which you can make some basic assumptions: steadily falling pressure means poor weather (storms, rain, etc) is incoming while steadily rising pressure denotes improving conditions. Also, steady pressure denotes stable air with little change expected over the course of the next two days. For more details, please reference the chart we’ve compiled above.

Adjusting for Sea Level

Last But Not Least

Remember above where we mentioned that differences in elevation result in different baseline air pressures? Well, when referencing the above chart you’re going to need to adjust to sea level. You can do this by matching the barometer’s reading to that reported by your local weather report. However, after continued use and note taking with your new barometer, you’ll begin to notice trends in conjunction with rising and falling action of the needle – often to the point where the numbers themselves will become irrelevant. It’s here where you’ll be able to provide short-term weather forecasts from the safety of your own home without having to rely on your phone or television. Isn’t autonomy great?

Now, It's Time To Understand That Watch Bezel

From a barometer to a watch bezel, these are skills we feel every man should know. So now that you have the first leg under your belt, how about brushing up on reading a watch bezel properly?